In recent years, as environmental issues are being discussed, attention is being given to hybrid vehicles, electric vehicles and other similar motored vehicles. These vehicles have mounted therein a motive power source implemented by a motor and an electric power source therefor implemented by a secondary battery, a capacitor or a similar power storage device.
Generally, a secondary battery, a capacitor or a similar power storage device decreases in capacity when temperature decreases, and as a result they have a decreased charging/discharging characteristic. Accordingly, when the power storage device has a decreased temperature, ensuring that the power storage device has a sufficient charging/discharging characteristic requires rapidly increasing the temperature of the power storage device.
Japanese Patent Laying-open No. 2004-15866 discloses a charging/discharging control device capable of increasing the temperature of a secondary battery in a short period of time. The charging/discharging control device charges/discharges a power storage device to attain a state of charge (SOC) allowing the secondary battery to generate heat in a maximum amount. The secondary battery can be increased in temperature in a short period of time and the power storage device's dischargeable output and chargeable input can be improved in a short period of time.
A power storage device mounted in a motored vehicle is often charged from a power supply external to the vehicle at midnight, since midnight electric power is inexpensive. At midnight, however, temperature is low, and the power storage device has a decreased charging characteristic. The power storage device may thus not be charged to its fully charged state.
The Japanese Patent Laying-open No. 2004-15866 describes that, to increase the secondary battery's temperature, the charging/discharging control device charges/discharges the power storage device to attain an SOC that allows the secondary battery to generate heat in a maximum amount. When a power storage device is charged from a power supply external to the vehicle, however, it is done so to its fully charged state, and the SOC cannot be controlled to have a value allowing the secondary battery to generate heat in a maximum amount. As such, the technique described in Japanese Patent Laying-open No. 2004-15866 cannot be used to charge a power storage device from a power supply external to a vehicle.